1. Field of the Invention
This invention relates generally to automatic flight control systems for aircraft and more particularly to autopilot apparatus for automatically capturing a preselected altitude under various automatic control engage conditions including abnormal conditions.
2. Description of the Prior Art
Automatic flight control systems for aircraft have for many years included the capability of causing the aircraft to automatically capture a predetermined altitude or an altitude that is preselected by the human pilot. These systems have performed with various degrees of success but were highly dependent upon performance limitations dictated by predetermined initial engage conditions, maneuver limits, passenger comfort, time of initiation of the automatic capture mode and so forth. In other words, the prior systems performed adequately only if all narrowly constrained engage prerequisites were met. For example, typical prior art automatic altitude capture systems attempted to precisely control the aircraft to a predetermined flight path in accordance with a predetermined control law in a continuous closed-loop fashion. Therefore, if for some operational reason, the aircraft was outside predetermined capture limits imposed by the control law, sometimes referred to as the capture window, automatic capture was impossible or if possible was accomplished in an extended oscillating manner and possibly exceeding desired g limits. For example, there may be instances wherein the pilot was operating the aircraft in a flight director mode and he desired to engage the automatic pilot during a capture maneuver. If this engagement took place under conditions wherein a substantial flight director error existed such that the aircraft was outside the automatic capture limits, a successful, smooth automatic capture within predetermined g limits could not be accomplished.
One such typical prior art automatic altitude capture apparatus is disclosed in the present assignee's U.S. Pat. No. 3,240,446 to H. Miller. With this apparatus, a smooth asymptotic autopilot engaged capture assumes autopilot engagement at altitude selection and that all initial conditions are fully satisfied. Another typical prior art automatic altitude capture apparatus is disclosed in the present assignee's U.S. Pat. No. 4,114,842 to Hofferber et al. This apparatus is an improvement over Miller in that it provides for a g-limited capture even though the pilot, occupied with other cockpit duties, inadvertently fails to arm the capture mode within the capture engage window. This is referred to a late capture engage and is accomplished by adjusting, within a predetermined g restraint, the window limits in accordance with existing aircraft position and rate conditions.